The present invention relates to a heat treatment method for providing different mechanical characteristics for a single treated piece.
When different parts of a single piece must have different mechanical or metallurgical characteristics, in accordance with the most widely used prior art technique, after a plurality of materials each having different chemical characteristics are independently heat-treated, the treated materials are rigidly coupled to each other by welding or the like. Exceptionally, there have been some conventional methods using only heat treatments to satisfy the above described requirement. For example, in one of these methods, local quenching is employed with which one side of a piece requiring hardness and strength is heated up to a preferred quenching temperature and thereafter water-quenched. Such a quenching method is the same as the more generally used quenching methods except that here only one side of the piece is quenched. However, the parts to which the heat treatment is not applied may have undesirable mechanical characteristics in comparison with the quenched parts, and the heat treated parts only be needed to be used in the site. Therefore, overall products produced using this method often cannot be used under severe circumstances.
A method has also been heretofore used with which tempering rather than quenching has been used to somewhat change mechanical strength locally. Temperatures are changed locally for the tempering procedure. However, since metallugical characteristics of steel mainly depend on the heating temperature used in quenching and the cooling speed employed and as tempering generally involves the use of smaller temperature changes, the mechanical strength of parts so treated are only slightly changed, that is, the metallurgical characteristics of each part cannot be greatly changed.
There have been some requirements that high strength at a raised temperature be imparted to one part of a product while at the same time low temperature toughness be provided to the other part of the same product, the two characteristics being metallugically quite contrary to each other. It has heretofore not been possible to satisfy such requirements as there has been no method available to solve the problem by using heat treatment alone.